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A couple whose sons are colourblind with $AB$ blood group, identify the parents from the following..
Mother colourblind with $A$ blood group, and father normal with blood group-$B$
Mother normal with blood group-$A$, and father colourblind with blood group-$B$
Mother colourblind with blood group-$B$, and father normal with blood group-$B$
Mother normal with blood group-$A$, and father colourblind with blood group-$B$
Solution
Colour blindness is caused by recessive sex-linked gene carried by $X$-chromosome. So, in male, one recessive gene is sufficient for its expression, $i.e.,$ $|X^{C} Y|$ but female needs two recessive gene $|X^{C} X^{C}|$ for the expression of colour blindness.
Mother will colourblind with $A$ blood group and father normal with blood group-$B$ as $-i$
So, all the sons will be colourblind with $AB$ blood group..
